If this product us a yeast raised product  I think the problem with it is the amount of the equilibrium relative humidity in that moisture in the product migrates to the sugar coating making it soft or the product is absorbing moisture from the environment 

Has Maria tried using humectants for her product or tried reducing the water activity of the product  she can also use barrier films to limit the product from  absorbing moisture from the environment, 

If she is having trouble with mouldings how about trying a combination of (propionic acid +potassium sobate + citric acid ) this gives a synergistic effect on mold and covers a wide range of bread mold including bacteria 

Sorbic is not good for yeast raised products but balchem has  an encapsulated bit of it it can be used in yeast products